1. Field of the Invention
The present invention relates to an apparatus and method for detecting a vehicle location in a navigation system, wherein a vehicle location can be precisely estimated using detection signals from sensors installed at a vehicle and then map-matched on a digital map so as to be displayed on a display screen.
2. Description of the Related Art
As the number of vehicles such as cars continuously increases, traffic congestion becomes more critical. Particularly, there is a serious problem in that the rate of increase in the number of vehicles is much higher than the rate at which roads and other infrastructures are being expanded. One recent study found that, in the major urban areas of the United States, the total cost of traffic congestion exceeds $47.5 billion per year. Traffic congestion wastes more than 14.35 billion liters of fuel and 2.7 billion hours of work time per year. These numbers have continued to increase by 5% to 10% per year through the 1990s.
As one of solutions to such traffic congestion, attention has been paid to a navigation system for guiding a travel path of a vehicle using a global positioning system (GPS). GPS is a satellite-based navigation system developed by the U.S. Department of Defense, wherein a plurality of GPS satellites arranged on geostationary orbits over the Earth transmit navigation messages, respectively, and a GPS receiver receives navigation messages transmitted by at least four GPS satellites among the navigation messages transmitted by the plurality of GPS satellites, so as to detect the distance between the GPS satellites and the GPS receiver and position vectors of the GPS satellites using the received navigation messages and to calculate a position vector of the GPS receiver. That is, it is possible to precisely detect 3D coordinates of the receiver that receives the navigation messages from the GPS satellites anywhere over the world.
In a navigation system using GPS, a GPS receiver receives navigation messages transmitted by a plurality of GPS satellites so as to detect a location at which the navigation messages are received, i.e. a vehicle location, using the received navigation messages, and the detected vehicle location is map-matched on a digital map so that the digital map and the vehicle location can be displayed on a display screen.
Therefore, a vehicle user can check a current vehicle location, a shortest path from the current location to a destination, and the like using the navigation system. Further, under the guidance of the navigation system, the vehicle user can beforehand plan a travel path along which a vehicle travels and be guided so that he/she moves the vehicle along the planned travel path, resulting in efficient use of road systems.
However, the navigation system can precisely detect a vehicle location only at an area where the GPS receiver can correctly receive the navigation messages transmitted by the GPS satellites. That is, since GPS detects a vehicle location using the GPS receiver that receives the navigation messages transmitted by the GPS satellites, it may not detect coordinates of a vehicle location or merely can obtain very incorrect results of detection of the coordinates at areas, including the interiors of tunnels, thickly-wooded forests or downtown areas surrounded by skyscrapers, where the GPS receiver cannot correctly receive the navigation messages transmitted by the GPS satellites.
Thus, a deduced reckoning system for deducing a vehicle location by detecting the travel distance and direction of a vehicle has been employed at areas where a vehicle location cannot be calculated since a GPS receiver cannot correctly receive navigation messages from GPS satellites.
The deduced reckoning system continues to measure the travel distance and direction of the vehicle from its initial location by using sensors including an odometer for detecting the number of revolutions of a driving wheel of the vehicle, and a gyroscope for detecting a travel angle difference of the vehicle. The vehicle location is consecutively estimated from the distance traveled and the travel direction by continuously integrating the number of revolutions of the driving wheel and the travel angle difference that have been measured. Since such a deduced reckoning system should initially know the coordinates of a reference location of the vehicle, it is also called “dead-reckoning.”
However, the deduced reckoning system has disadvantages in that the initial location of the vehicle should be set correctly, and an estimated location of the vehicle is incorrect due to an accumulation of measurement errors inherent to the sensors if the vehicle travels for a long time during a process of deducing the vehicle location using the deduced reckoning system.
To solve these problems, there has been developed a hybrid navigation system. The hybrid navigation system comprises a GPS receiver for receiving a navigation message, and sensors installed on a vehicle, such as a gyroscope and an odometer for detecting the travel angle difference and traveled distance of the vehicle. If the value of dilution of precision (DOP) of the navigation message received by the GPS receiver is less than a predetermined threshold, a vehicle location is detected based on the received navigation message and the detection signals from the sensors. If the value of dilution of precision (DOP) of the navigation message received by the GPS receiver is equal to or greater than the predetermined threshold, the vehicle location is detected by using only the detection signals from the sensors without using the received navigation message.
When the GPS receiver detects a vehicle location by receiving navigation messages transmitted by a plurality of GPS satellites, DOP as a geometrical error is produced according to the relationship of arrangement between the GPS satellites, which have transmitted the navigation messages received by the GPS receiver, with respect to the position of the GPS receiver. The value of DOP becomes small if the GPS satellites are arranged uniformly with respect to the position of the GPS receiver, but large if the GPS satellites are not arranged uniformly. The GPS receiver detects the value of DOP according to the relationship of arrangement between the relevant GPS satellites from which the GPS receiver receives the navigation messages. If the value of DOP according to the relationship of arrangement between the relevant GPS satellites from which the GPS receiver receives the navigation messages is less than 2, this is an excellent case. If the value of DOP ranges from 2 to 3, this is a good case. If the value of DOP ranges from 4 to 5, this is a regular case. If the value of DOP is equal to or greater than 6, this cannot be utilized due to many errors in the position of the GPS receiver detected from the received navigation messages.
The hybrid navigation system determines based on the value of DOP whether the navigation messages received by the GPS receiver are used. If the value of DOP is less than a predetermined threshold, e.g., 5, the vehicle location is detected using both the received navigation messages and the detection signals from the sensors installed on the vehicle. If the value of DOP is equal to or greater than 5, the vehicle location is detected using only the detection signals from the sensors without using the received navigation messages.
However, in the case where the hybrid navigation system detects a current vehicle location using only the detection signals from the sensors since the value of DOP of a navigation message received by the GPS receiver is equal to or greater than the predetermined threshold, the detection of the current vehicle location is iterated using a vehicle location, which has been detected using just previous detection signals from the sensors, as a reference location. Thus, errors in the vehicle location detected using the detection signals from the sensors are continuously accumulated. Accordingly, there is a problem in that the difference between an actual vehicle location and a vehicle location detected using the detection signals from the sensors increases continuously with time.
That is, on the assumption that a vehicle travels along links between nodes L1, L2 and L3 as shown in FIG. 1, vehicle locations P1 and P2 are precisely detected using navigation messages with the values of DOP less than a predetermined threshold received by a GPS receiver, and vehicle locations are then detected using only detection signals from sensors installed on the vehicle after the values of DOP of received navigation messages become equal to or greater than the threshold, location P2 is detected and current vehicle location P3 is then detected using the detection signals from the sensors with respect to location P2. Thereafter, current vehicle location P4 is detected with respect to vehicle location P3 that has been detected just previously. Iteration of such processes results in detection of locations P5, P6, P7, P8 and P9.
However, the detection of a vehicle location using only the detection signals from the sensors, i.e. a travel angle difference and traveled distance, contains a little error in the detected vehicle location in view of characteristics of the sensors. Such a prior art has a problem in that since a current vehicle location is detected with respect to a just previously detected vehicle location, errors in vehicle locations detected using the detection signals from the sensors are continuously accumulated and thus the difference between an actual vehicle location and a detected vehicle location increases with time.
Further, when the traveled distance of the vehicle is estimated from the detection signals from the sensors in the prior art, the distance traveled per unit time is estimated by multiplying the number of pulse signals, which are generated from an odometer depending on a travel of the vehicle irrespective of a travel speed of the vehicle, by a predetermined distance conversion coefficient. Thus, there is a large difference between an actual vehicle location and a detected vehicle location according to the travel speed of the vehicle.
That is, when the vehicle travels, different frictional forces are produced between driving wheels and a road according to the travel speed of the vehicle. Due to this phenomenon, even though the vehicle travels by an identical distance, the number of pulse signals generated by the odometer varies according to the travel speed of the vehicle.
However, since the traveled distance of the vehicle is estimated by multiplying the number of pulse signals, which are generated from the odometer, by only the predetermined distance conversion coefficient in the prior art, a large difference occurs between an actual vehicle location and a detected vehicle location according to the travel speed of the vehicle.
Further, the travel angle difference of the vehicle detected by an output signal from the gyroscope cannot correctly reflect a 3D gradient of a road on which the vehicle travels. Moreover, the traveled distance of the vehicle detected by the odometer cannot also reflect a fore-and-aft gradient of a road on which the vehicle travels. These become causes of the occurrence of errors in the determination of a vehicle location, resulting in a large difference between the traveled distance and a traveled distance on a plane of an actual map.
Although there has been proposed a method using a cumulative inclinometer to reduce such errors, measurement errors are continuously accumulated in the cumulative inclinometer. Thus, an estimated vehicle location becomes incorrect with time. Further, at a junction of an overpass and a surface street, what road a vehicle enters or exits from cannot be correctly determined. Therefore, there is a problem in that errors occur in performing map-matching of a vehicle location on a digital map.